Ah! Okay. I've never seen a plastic sill before, but originally this house didn't have boxing trim, either -- I don't think they used it much mid-century. I had the window company trim the front ones with wood when I had them replaced, because I prefer it to having the drywall go right up to the window.

I actually love MDF... as a painting support. I seal and gesso it, and it makes a lovely smooth, light surface to paint on. It's very dimensionally stable, so I don't have to worry about the painting cracking or warping. But my paintings aren't down where I kick them. :P

I'd probably have to look at MDF trim if I wanted to trim out the whole house at once, but since I'm planning to do long-range piecemeal I can afford paint-grade wood, instead.